Wavelength division multiplexing is a technique that may be used to improve the efficiency and increase the utility of lightwave systems by expanding the information transmission rate without increasing the bit rate. Wavelength division multiplexing could lead to lower system costs through simplified circuitry and a reduction in the number of required repeaters, and could increase the possibility of adding system capabilities such as two-way transmission or simultaneous transmission of analog and digital signals.
Several passive devices for wavelength division multiplexing the demultiplexing have been reported in the prior art. In addition, a new photodetector demultiplexing structure that uses stacked epitaxial indium, gallium, arsenide phosphide layers to detect and demultiplex two wavelength bands has been disclosed in a recent copending application. See application Ser. No. 969,346 entitled "Demultiplexing Photodetector", filed on Dec. 14, 1978, by J. C. Campbell and T. P. Lee. The device disclosed in the copending application has also been reported in the article entitled "Dual-Wavelength Demultiplexing InGaAsP Photodiode" by J. C. Campbell et al, Applied Physics Letters, 34(6), Mar. 15, 1979, pp. 401-402.
The demultiplexing photodetector disclosed in the above-identified copending application and article by J. C. Campbell et al is also shown as FIG. 1 in the drawings of this application. As indicated in FIG. 1, the prior art device consists of five epitaxial layers grown on a &lt;111&gt; indium phosphide substrate. Two of these layers 103 and 105 denoted as Q.sub.1 and Q.sub.2 respectively in FIG. 1 are InGaAsP layers that have different crystal compositions and therefore different bandgap energies. These layers are positioned such that the incoming light 150 first strikes quaternary layer 105 (Q.sub.2) having the larger bandgap energy. Photons in the incoming light having an energy less than the bandgap of the indium phosphide window layer 106 but greater than that of the Q.sub.2 layer 105 are absorbed in that layer. The photogenerated carriers are collected by the pn junction 109 in Q.sub.2 and appear as a signal V.sub.2 across the load impedance 115. Similarly, the Q.sub.1 layer 103 absorbs photons whose energy is less than the bandgap of Q.sub.2 layer 105 but greater than that of Q.sub.1 layer 103 thereby giving rise to a signal V.sub.1 across the load impedance 116. In this prior art device, indium phosphide layer 106 is also necessary in order to prevent a recombination of holes and electrons that would otherwise take place at the surface of the quaternary layer 105.
Experiments with the prior art device shown in FIG. 1 have revealed that the operation of the device is limited to wavelengths that are less than or equal to about 1.25 micrometers. Attempts to make a device that worked at longer wavelengths were unsuccessful.